Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. . . . The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997.
The known fluoropolymers include in particular fluoroelastomers and fluorothermoplasts. Such fluoropolymers are generally copolymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as for example hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P).
Examples of fluoroelastomers include for example copolymers of TFE and PVE and copolymers of VDF and HFP. The fluoroelastomers may also contain cure site components so that they may be cured if desired. Applications of fluoroelastomers include for example coatings, use as gaskets and seals as well as use as polymer processing aids (PPA). A commercially available processing aid includes for example copolymers of VDF and HFP available from Dyneon LLC under the brand DYNAMAR™ PPA.
Examples of fluorothermoplasts include semicrystalline copolymers of TFE and E (ETFE), copolymers of TFE and HFP (FEP), copolymers of TFE, HFP and VDF (THV) and perfluoroalkoxy copolymers (PFA). Examples of applications of fluorothermoplasts include for example coating applications such as for example for coating outdoor fabric and use as insulating material in wire and cable insulation. In particular ETFE copolymers have desirable properties as insulating material. Further applications of fluorothermoplasts include making of tubes such as for example fuel hoses, extrusion of films and injection molded articles. The extruded fluorothermoplastic articles, in particular films may further be subjected to an e-beam radiation to partially cure the fluorothermoplast.
Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. Nos. 3,855,191, 4,439,385 and EP 649863; aqueous emulsion polymerization as disclosed in e.g. U.S. Pat. Nos. 3,635,926 and 4,262,101; solution polymerization as disclosed in U.S. Pat. Nos. 3,642,742, 4,588,796 and 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used for the stabilization of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally good stability over a long period of time is obtained.
An aqueous emulsion polymerization wherein no surfactant is used has been described in U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381 to generally produce homo- and copolymers of chlorotrifluoroethylene (CTFE). For example, WO 97/17381 discloses an aqueous emulsion polymerization in the absence of a surfactant wherein a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization and whereby the initiator system is added in one or more further charges during the polymerization. However, the aqueous emulsion polymerization process disclosed there has the disadvantage that a dual feed of reducing agent and oxidizing agent is required, making the process more cumbersome. This means in practice, for example, that additional feeding lines and control devices are needed and the dual feed inevitably increases the risk of failures during the polymerization. More importantly, WO 97/17381 mainly relates to the control of the latex particles size as well to the particle size distribution in the absence of fluorinated emulsifier, the control of the molecular weight as well as the molecular weight distribution (MWD) is not concerned herein. WO 97/17831 also discloses the optional use of chain transfer agents and discloses in one of the examples an aqueous emulsion polymerization process using ethyl acetate as a chain transfer agent. It was however found that the use of ethyl acetate results in the formation of water-soluble fluorinated substances, which is undesirable.
The aqueous emulsion polymerization process in the presence of fluorinated surfactants is a desirable process to produce fluoropolymers because it can yield stable fluoropolymer particle dispersions in high yield and in a more environmental friendly way than for example polymerizations conducted in an organic solvent. However, for certain applications, the fluoropolymers produced via the aqueous emulsion polymerization process may have undesirable properties relative to similar polymers produced via solution polymerization. For example, purity is required for polymers used in applications with food contact, and in particular the presence of extractables (e.g., fluorinated surfactants and other low molecular weight fluorinated compounds) is highly regulated. Furthermore, fluorinated surfactants typically used in aqueous emulsion polymerization such as perfluoro octanoic acid or perfluoro sulfonic acids are expensive and are considered as environmental concern nowadays. It is therefore desirable to run aqueous emulsion polymerizations in the absence of surfactants without however compromising the properties of the polymers resulting.
It would also be desirable to improve the aqueous emulsion polymerization process so that also fluoropolymers of higher quality can be produced meeting the needs of demanding applications. In particular, it would be desirable to improve properties such as the mechanical and physical properties of the resulting polymer, the purity level, reducing the amount of extractable substances, reduce discoloration, improved processability and improving performance of the fluoropolymer such as for example the compression set and permeation in case of a curable fluoroelastomer.